The belly button, it turns out, is one of the least-studied parts of the human body. But one of its great mysteries may now have been solved – why most people have an innie, or concave belly button.

I was a weird kid – I used to be very jealous of people who were left-handed, and desperately wanted an outie belly button (I don’t know what was going on with me either). Like 90% of people, though, ...